I have done oxygen plasma treatment on PDMS material and got bonded and free C=O functional groups at 1635 and 1730 cm-1 respectively in FT-IR spectra.

I kept the samples in ambient environment for 5 days to study the aging behavior.

In the 5 days aging duration, concentration of above mentioned two peaks increase and decreases randomly. I have attached the file showing this data.

What physical phenomena is responsible for this kind of random change in concentration?

While measuring contact angle and surface energy, found that polar part of the SE linearly decays with time and so water contact angle show hydrophobic recovery.

Whereas dispersive part of SE remains nearly constant with time and so non polar liquid contact angle also does not change much.